There are five brands out there:
1: Medela. The draws on these ones are the ability to pump directly in them if you want to and the claim by the company that the double walls help prevent freezer burn. I have never had one of these leak in almost 7 years of pumping and storing milk. But they are expensive, and I find them really quite difficult to pour out of. I also find that pumping right into them is silly...You almost can't move while doing so because it's so easy to spill the milk from them!
2. Lansinoh. What I like about these is that they can hold a lot more milk. I put 6-7 oz in these and it does just fine. I have found they will split if you drop them while frozen and then they will leak. I have a good amount of stash in these because I find them about every 3 months on sale somewhere. I put them in a ziplock bag in the freezer to help prevent freezer burn (as well as removing all air from the bag before sealing it.)
3. Honeysuckle. I find them to be almost just like Lansinoh, but they have to be ordered (which is OK if I remember I need them early enough!) and are biodegradeable, which is very cool. I think the bottle image on the top is totally stupid. I think the plastic is more durable than Lansinoh. I dropped a couple frozen bags and they didn't split.
4. Simplisse. I got a box of these at BRU a few weeks ago, just to try, but I haven't used them yet as I still had about 100 Lansinoh bags at the time. They are very like Lansinoh, actually, only they were significantly less expensive. I've only seen them a couple times at the store. I'm sure they could be ordered. They are a pretty pink.
5. Gerber. Owned by Nestle if you care about that kind of stuff, which is why I don't buy them. But they were OK IME. I haven't used one in years and years though.
I'm sure there's some other brands available as well.